1. Field of the Invention
The present invention relates in general to a system for receiving a broadcast signal such as a television signal, and more particularly to a broadcasting reception system which is capable of removing a ghost signal from a broadcast video signal.
2. Description of the Prior Art
Generally, a ghost signal is contained in a broadcast signal received by a television receiver. For this reason, a broadcast signal is normally transmitted from a broadcasting station, with a reference signal being contained in a main video signal of the broadcast signal for the purpose of removal of the ghost signal. The reference signal is, typically, contained in the video signal at the rate of one line per field. Alternatively, the reference signal may be contained in the video signal at the rate of one line every eight lines or at the rate of one line per frame. In an NTSC television broadcasting system, each screen is referred to as a frame, which consists of two fields and has 525 scanning lines.
Referring to FIG. 1, there is shown a block diagram of a conventional ghost signal removal apparatus for a broadcast reception system such as a television receiver. As shown in FIG. 1, the conventional ghost signal removal apparatus includes an analog/digital (A/D) converter 1 for converting a main video signal x(t) of a received television signal into a corresponding digital signal x(n).
A transversal filter 2 is provided for removing from the digital signal x(n) a ghost signal component appearing at the front of a main video signal, and a transversal filter 3 is provided for removing a ghost signal component appearing at the rear of the main video signal. A digital/analog (D/A) converter 4 converts a final video signal y(n), already processed for ghost signal removal into an analog signal. A subtracter 5 subtracts an output signal from the transversal filter 3 from an output signal from the transversal filter 2, and outputs the resulting signal as the final video signal y(n), processed for ghost signals removal, to the D/A converter 4.
A memory 6 stores a reference signal x'(n) which is contained in the digital video signal x(n) from the A/D converter 1. A memory 9 stores the final video signal y(n) from the subtracter 5. Additionally, a memory 12 stores an error signal e(n).
A filter coefficient generator 7 executes a least mean square (LMS) algorithm using the reference signal x'(n) stored in the memory 6 and the error signal e(n) stored in the memory 12 to obtain a filter coefficient A(n) for the transversal filter 2. A filter coefficient generator 8 executes a least mean square (LMS) algorithm using the final video signal y(n) stored in the memory 9 and the error signal e(n) stored in the memory 12 to obtain a filter coefficient B(n) for the transversal filter 3.
A memory 10 stores a predetermined ghost removal reference signal r(n) which is the same as a ghost removal reference signal transmitted from a broadcasting station. A subtracter 11 subtracts the predetermined reference signal r(n) from the final video signal y(n) and outputs the resulting signal as the error signal e(n).
The operation of the conventional ghost signal removal apparatus with the above-mentioned construction will hereinafter be described.
The broadcast signal transmitted from the broadcasting station through an antenna is reflected by objects such as buildings, mountains, etc., and the original and reflected signals are received by television receivers with time differences among them. The video signal x(t) component of the received broadcast signal can be defined by the following equation (1) with respect to time t: EQU x(t)=. . . a.sub.-2 u(t-2)+a.sub.-1 u(t-1)+u(t)+a.sub.1 u(t+1)+a.sub.2 u(t+2)+. . . (EQUATION 1)
where, a&lt;1.
Herein, the u(t) term represents the main video signal component as shown in a diagram in FIG. 2. The diagram illustrates a screen which contains the main video signal component and ghost signal components of a received broadcast signal. The terms positioned in the equation to the left of the u(t) term represent the ghost signal components appearing at the front of the main video signal, and the equation terms positioned to the right of the u(t) term represent the ghost signal components appearing at the rear of the main video signal.
The video signal x(t) of the received television signal is converted into the digital signal x(n) (where, n is the sample number) by the A/D converter 1, which then applies the digital signal x(n) to the transversal filter 2.
At this time, only the reference signal x'(n) component of in the digital signal x(n) from the A/D converter 1 is stored in the memory 6. The filter coefficient generator 7 generates the filter coefficient A(n) for the transversal filter 2 by performing the LMS algorithm using the reference signal x'(n) stored in the memory 6 and the error signal e(n) stored in the memory 12. The transversal filter 2 adjusts its filter coefficient in accordance with the filter coefficient A(n) from the filter coefficient generator 7 and filters the digital signal x(n) from the A/D converter 1 on the basis of the adjusted filter coefficient. The transversal filter 2 outputs a video signal with the ghost signal component at the front of the main video signal removed. The video signal from the transversal filter 2 is fed to the subtracter 5.
The transversal filter 3 receives the final video signal y(n) as an input and adjusts its filter coefficient in accordance with the filter coefficient B(n) from the filter coefficient generator 8. The transversal filter 3 then filters the final video signal y(n) on the basis of the adjusted filter coefficient, thereby to output the video signal in which has been removed the ghost signal component at the rear of the main video signal. The video signal from the transversal filter 3 is also fed to the subtracter 5.
The filter coefficient generator 8 obtains the filter coefficient B(n) for the transversal filter 3 by executing the LMS algorithm using the final video signal y(n) stored in the memory 9 and the error signal e(n) stored in the memory 12.
The subtracter 5 subtracts the output signal from the transversal filter 3 from the output signal from the transversal filter 2 and outputs the subtracted signal as the final video signal y(n), processed for ghost signal removal, to the D/A converter 4, which then converts the final video signal y(n) into the analog signal y(t).
The memory 6 stores the reference signal x'(n) for removal of the ghost signal, which is contained in the video signal transmitted from the broadcasting station, and then feeds the stored reference signal x'(n) to the filter coefficient generator 7 as a signal for use in execution of the LMS algorithm. The memory 9 stores the final video signal y(n) from the subtracter 5 and then feeds the stored final video signal y(n) to the filter coefficient generator 8 as a signal for use in execution of the LMS algorithm.
The subtracter 11 receives the final video signal y(n) from the subtracter 5 and the ghost signal removal reference signal r(n) from the memory 10 and generates the error signal e(n) for storage in the memory 12.
The memory 12 feeds the stored error signal e(n) to the filter coefficient generators 7 and 8 as a signal for use in execution of the LMS algorithm. The transversal filters 2 and 3 are a kind of adaptive digital filter.
Referring to FIG. 3, there is shown a block diagram of the transversal filters 2 and 3. As shown in this drawing, the transversal filter 2 includes a delay element 2a for delaying and storing the digital signal x'(n) inputted therein by a plurality of steps. Each of a plurality of gain adjustment taps 2b, adjusts a gain of a corresponding one of the signals stored by the plurality of steps in accordance with a corresponding one of elements of the filter coefficient A(n) from the filter coefficient generator 7. An adder 2c adds output signals from the plurality of gain adjustment taps 2b.
Similarly, the transversal filter 3 includes a delay element 3a for delaying and storing the digital signal inputted therein by a plurality of steps. Each of a plurality of gain adjustment taps 3b adjusts a gain of a corresponding one of the signals stored by the plurality of steps in accordance with a corresponding one of elements of the filter coefficient B(n) from the filter coefficient generator 8. An adder 3c adds the output signals from the plurality of gain adjustment taps 3b.
In the transversal filter 2, the delay element 2a delays the inputted digital signal or the video signal x(n) by n-1 to n-NF steps and sequentially stores the delayed NF samples. Each of the NF gain adjustment taps 2b reads a corresponding one of the stored NF samples and adjusts the gain of the read sample in accordance with a corresponding one of the elements a.sub.1 to a.sub.NF of the filter coefficient A(n) which are generated by the filter coefficient generator 7 as a result of the execution of the LMS algorithm. The adder 2c adds the output signals from the gain adjustment taps 2b and outputs the added signal as the video signal from which the ghost signal component at the front of the main video signal has been processed for removal.
In the transversal filter 2, the delay element 3a delays the inputted digital signal or the fed-back final video signal y(n) by n-1 to n-NB steps and sequentially stores the delayed NB samples. Each of the NB gain adjustment taps 3b reads a corresponding one of the stored NB samples and adjusts the gain of the read sample in accordance with a corresponding one of the elements b.sub.1 to b.sub.NB of the filter coefficient B(n) which are generated by the filter coefficient generator 8 as a result of the execution of the LMS algorithm. The adder 3c adds the output signals from the gain adjustment taps 3b and outputs the added signal as the video signal from which the ghost signal component at the rear of the main video signal has been processed for removal.
The subtracter 5 subtracts the output signal from the transversal filter 3 from the output signal from the transversal filter 2 and outputs the resulting signal as the ghost signal-removed final video signal y(n), which can be defined by the following equation: ##EQU1## where, the ai and bj terms are the filter coefficients or gain adjustment coefficients which are adjusted as a result of the execution of the LMS algorithm, i and j are progression variables, and N is a positive integer.
The video signal in FIG. 1 can be expressed as the sample values as follows:
Namely, the reference signal x'(n) which is fed from the memory 6 to the filter coefficient generator 7 can be expressed by the following equation: EQU x'(n)=[x'(n)x'(n-1) . . . x'(n-NF)] (EQUATION 3)
The final video signal y(n) which is fed from the memory 9 to the filter coefficient generator 8 can be expressed by the following equation: EQU y(n)=[y(n-1) . . . y(n-NB)] (EQUATION 4)
The ghost signal removal reference signal r(n) which is stored in the memory 10 can be expressed by the following equation: EQU r(n)=[r(n)r(n-1)r(n-2) . . . r(n-NB)] (EQUATION 5)
The error signal e(n) which is outputted from the subtracter can be expressed by the following equation: EQU e(n)-r(n)-y(n) (EQUATION 6)
The filter coefficient generator 7 executes the LMS algorithm with respect to the inputted signals x'(n) and e(n) as follows: EQU A(n)=A(n-1)+2K.sub.1 e(n-1)x'(n-1) (EQUATION 7)
The filter coefficient generator 7 obtains the filter coefficient A(n) of the transversal filter 2 by executing the LMS algorithm defined by EQUATION 7. The elements a.sub.1 -a.sub.NF of the filter coefficient A(n) from the filter coefficient generator 7 are applied to the gain adjustment taps 2b in the transversal filter 2, respectively, and can be expressed by the following equation: EQU A(n)=[a.sub.0 (n)a.sub.1 (n)a.sub.2 (n) . . . a.sub.NF (n)].sup.T (EQUATION 8)
The filter coefficient A(n) is repeatedly obtained such that it is adaptive to the reference signals which are contained in the successively inputted video signals, for removal of the ghost signal components at the front of the main video signals in the transversal filter 2.
The filter coefficient generator 8 executes the LMS algorithm with respect to the inputted signals y(n) and e(n) as follows: EQU B(n)=B(n-1)+2K.sub.2 e(n-1)y(n-1) (EQUATION 9)
The filter coefficient generator 8 obtains the filter coefficient B(n) of the transversal filter 3 by executing the LMS algorithm defined by EQUATION 9. The elements b.sub.1 -b.sub.NF of the filter coefficient B(n) from the filter coefficient generator 8 are applied to the gain adjustment taps 3b in the transversal filter 3, respectively, and can be defined by the following equation: EQU B(n)=[b.sub.0 (n)b.sub.1 (n)b.sub.2 (n) . . . b.sub.NB (n)].sup.T (EQUATION 10)
The filter coefficient B(n) is repeatedly obtained such that it is adaptive to the reference signals which are contained in the successively inputted video signals, for removal of the ghost signal components at the rear of the main video signals in the transversal filter 3. In the LMS algorithms for the filter coefficients A(n) and B(n), K.sub.1 and K.sub.2 are inherent constants of the transversal filters 2 and 3 which are determined suitably to convergence speeds and stabilities of the transversal filters 2 and 3, respectively.
As indicated above, the conventional ghost signal removal apparatus comprises the transversal filters 2 and 3, which are adaptive digital filters, and the filter coefficient generators 7 and 8 which execute the LMS algorithm to obtain the filter coefficients A(n) and B(n) for the transversal filters 2 and 3, respectively, thereby providing for removal of the ghost signal components at the front and rear of the main video signal.
However, the conventional ghost signal removal apparatus has the following disadvantages:
First, since the LMS algorithms for removal of the ghost signal components at the front and rear of the main video signal are not performed, the ghost signal removal process is reduced in speed due to the resulting sequential executions.
Second, as connected for feedback operation, the transversal filter has an effect on the generation of the filter coefficient B(n), and the stability of the ghost signal removal operation accordingly cannot be assured.